Action and Reaction

34. The more recent thinkers consider motion and rest in bodies as 2 states of existence, in either of which every body would remain inert by its own nature, unless some external force urged it.

From this, the cause of motion and rest is the same as the cause of the existence of bodies.

Nor indeed does there seem to be another cause to be sought for the successive existence of a body in different parts of space than that from which the successive existence of the same body in different parts of time is derived.

But to treat of God, the Best and Greatest, the Creator and Preserver of all things, and to demonstrate in what manner all things depend on the supreme and true Being, although it is a most excellent part of human knowledge, nevertheless pertains rather to first philosophy, or metaphysics and theology, than to natural philosophy, which today is almost entirely contained in experiments and mechanics.

And so, natural philosophy either presupposes the knowledge of God, or borrows it from some higher science.

35. These things are less understood because some unjustly reject the mathematical principles of physics because they do not assign the efficient causes of things.

When nevertheless it truly pertains to physics or mechanics to hand down only rules, not efficient causes, of impulses or attractions, and to say it in a word, the laws of motions; and from these laid down, to assign the solution, not the efficient cause, of particular phenomena.

36. It will be very important to have considered what a principle properly is, and in what sense that word is to be understood among philosophers.

The true efficient and preserving cause of all things is most rightly called their source and principle.

But the principles of experimental philosophy are the foundations on which it rests, or the sources from which the knowledge of corporeal things is derived (I do not say existence, but knowledge), namely the senses from experience.

Similarly, in mechanical philosophy, the principles are said to be those in which the entire discipline is founded and contained, those primary laws of motion which, proven by experiments, have also been refined by reasoning and rendered universal.

These laws of motion are conveniently called principles, because from them both general mechanical theorems and particular explanations of phenomena are derived.

37. Then indeed can something be said to be explained mechanically, when it is reduced to these simplest and most universal principles, and is shown through accurate reasoning to be consistent and connected with them.

For once the laws of nature have been discovered, it must next be shown by the philosopher that from the constant observation of these laws, that is, from these principles, any phenomenon necessarily follows: this is to explain and solve phenomena, and to assign the cause, that is, the reason why they occur.

38. The human mind rejoices to extend and expand its knowledge.

But for this, general notions and propositions must be formed, in which particular propositions and cognitions are in a certain way contained, which are then believed to be understood only when they are deduced from the former by a continuous connection. This is very well known to geometers.

In mechanics also, notions, that is definitions, and primary and general statements about motion are put forward, from which [pg 515] afterwards more remote and less general conclusions are gathered by the mathematical method.

And just as by the application of geometrical theorems, the magnitudes of particular bodies are measured; so also by the application of universal mechanical theorems, the motions of any parts of the world system, and the phenomena depending on them, become known and are determined: and it is to this goal that the physicist should uniquely aim.

39. Geometers, for the sake of their discipline, devise many things which they themselves can neither describe nor find in the nature of things.

Similarly, the mechanician uses certain abstract and general terms. He and imagines in bodies force, action, attraction, solicitation, &c., which are most useful for theories and statements, as well as calculations about motion, even if they would be sought in vain in the very truth of things and actually existing bodies, no less than those things which are imagined by geometers through mathematical abstraction.

40. In truth, by the aid of the senses we perceive nothing but effects or sensible qualities, and corporeal things entirely passive, whether they are in motion or at rest; and reason and experience suggest that nothing active exists besides mind or soul.

Whatever else is imagined must be considered to be of the same kind as other hypotheses and mathematical abstractions: this must be deeply impressed upon the mind. If this is not done, we can easily relapse into the obscure subtlety of the Scholastics, which for so many centuries, like a dreadful plague, has corrupted philosophy.

41. The mechanical principles and the universal laws of motions or nature, happily discovered in the last century, and treated and applied with the aid of geometry, have brought wonderful light into philosophy.

But metaphysical principles and the real efficient causes of motion and the existence of bodies or corporeal attributes in no way pertain to mechanics or experiments; nor can they shed light on them, except insofar as, being known beforehand, they serve to predefine the limits of physics, and in that way to remove difficulties and extraneous questions.

42. Those who seek the principle of motion from spirits understand by the word “spirit” either a corporeal or an incorporeal thing. If a corporeal thing, however subtle, the difficulty still returns; if incorporeal, however true that may be, nevertheless it does not properly pertain to physics.

But if anyone extends natural philosophy beyond the limits of experiments and mechanics, so that it also comprehends the knowledge of incorporeal and unextended things, this broader acceptance of the word admits the treatment of the soul, mind, or vital principle.

But it will be more convenient, according to the use now almost received, to so distinguish between the sciences that each is circumscribed by its own boundaries, and that the natural philosopher is entirely concerned with experiments, the laws of motion, and mechanical principles, and the reasonings drawn from them; but whatever he puts forth about other things, let him attribute it to some higher science.

From the known laws of nature, the most beautiful theories and even mechanical practices useful for life follow. But from the knowledge of the Author of nature himself, considerations arise that are indeed far more excellent, but are metaphysical, theological, and moral.